Laminated products obtained by bonding a soft polyvinyl chloride (PVC) film or sheet and a urethane foam by heat or with an adhesive have a satisfactory appearance, toughness, a soft touch, and a cushioning effect and have been widely used as cushioning materials for furniture, outer covering materials for bags and shoes, automobile linings, e.g., door liners and ceiling coverings, and the like.
However, soft PVC of these laminates involves various problems arising from a plasticizer used therein for softening. For example, when soft PVC is in contact with other materials such as plastics, copied or printed matters, etc., the plasticizer migrates thereto to cause contamination of both soft PVC and the contacting materials. Further, the plasticizer is scratched out on contact with human bodies or clothing, which results in hardening or cracking of the soft PVC. When soft PVC is used for a long time especially in a high temperature as in car interiors, the plasticizer is evaporated to cause hardening or cracking of the PVC sheet. Moreover, soft PVC undergoes deterioration by heat or light and is liable to change its color to yellowish brown.
In the light of these defects of conventionally employed soft PVC, soft polyolefin resins have been extending use in place of soft PVC.
Cushioning covering materials-comprising a soft polyolefin resin have been produced by molding a soft polyolefin resin into a film or a sheet by means of a T die extruder or a calendering machine simultaneously with heat bonding with a 10 to 40-fold expanded sheet of a polyolefin resin, e.g., polyethylene or polypropylene.
However, covering materials having a highly expanded polyolefin sheet have the following disadvantages:
(1) Since an expanded foam of polyolefins, e.g., polyethylene and polypropylene, undergoes compression set, it does not completely recover even on relieving a compressive force when it is deformed to a great degree, or for a long time, or repeatedly. For example, compression set of a polyolefin foam as measured in accordance with JIS-K6767 (Method of Compression Set Test, deformation: 25%) is shown in Table 1 below in comparison with that of a urethane foam.
TABLE 1 ______________________________________ Compression Set Plastic Foam (%) ______________________________________ Polyethylene foam 3-10 Polypropylene foam 5-15 Urethane foam 0-1.5 ______________________________________
(2) Production of polyolefin foams requires at least a step of mixing a blowing agent with a polyolefin resin and extruding, a step of crosslinking, and a step of expansion and winding and thus entails a high cost.
On the other hand, a soft urethane foam which has been used in soft PVC laminates is superior to polyolefin foams in the following points.
(1) A soft urethane foam, even when deformed to a great degree, or for a long time, or in a repeated manner, nearly 100% recovers due to its small compression set.
(2) Production is completed simply by mixing liquid materials including a polyfunctional polyol, a diisocyanate, a catalyst, water, additives, etc. Solidification by crosslinking and expansion simultaneously proceed and, after completion of expansion, the expanded body is required only to be fabricated into a sheet. Therefore, an expanded body having a higher degree of expansion can be obtained through a smaller number of steps as compared with polyolefin foams. The cost per unit volume is about half the cost of the highly expanded polypropylene foam and about 1/1.5 that of the highly expanded polyethylene foam.
Accordingly, lamination of a soft urethane foam and a soft olefin resin film or sheet would provide the best cushioning material. These two materials, however, are not at all compatible with each other so that they cannot be laminated by application of heat but by using an adhesive. Namely, a laminate is produced, without any other alternative, by a process comprising first preparing a soft olefin resin sheet, re-heating the sheet with a two-pack crosslinkable urethane adhesive applied thereon, and then adhering a soft urethane foam thereto. Such a process not only entails an increased number of steps and an increased cost but also finds difficulty in obtaining a perfect laminate free from layer separation.